1. Field of the Invention
The present invention relates to an optical imaging device which irradiates the subject with low coherence light, to produce a tomogram of the subject, from data on light scattered by the subject.
2. Description of Related Art
Recent U.S. Pat. No. 5,321,501, for example, discloses a device which can obtain optical information from the inside of the tissue, in order to diagnose the biological tissue, and which performs optical coherence tomography (OCT) of interference type, using low coherence light, to produce a tomogram of the subject. U.S. Pat. No. 5,321,501 also discloses an optical probe which has a flexible insertion unit to be introduced into the patient""s body cavity, and which enables the doctor to introduce the insertion unit into a blood vessel, using an optical probe that includes a single mode fiber for sending low coherence light to the inside so that the doctor can observe the inside of the patient""s body cavity, using a catheter or endoscope. When introduced into the patient""s body cavity, the optical probe is necessarily bent according to the tortuosity of the body cavity. Because of the bending, stress-induced birefringence is induced in the optical fiber which varies according to the extent of the bending. In OCT, the doctor uses an interference signal of a reference beam with reflection from the subject, to observe the subject. It is necessary, generally, to orient polarization of the reflection from the subject, toward that of the reference beam so that interference with the reference beam may be maximized. The stress-induced birefringence varies depending on the bending in the patient""s body cavity; polarization, therefore, varies depending on the bending. As introduced into the patient""s body cavity, the insertion unit varies interference contrast. The doctor may turn an integrated irradiation optical system that includes the single mode fiber. Every rotation of the bent system, particularly in such cases, causes a great variation in the stress-induced birefringence of the fiber. Interference contrast varies, so that detection sensitivity greatly varies depending on the direction of rotary scanning. Such a great variation is a problem.
xe2x80x9cPolarization-insensitive fiber optic Michelson interferometerxe2x80x9d (Electr. Lett. Vol. 27, pp. 518-519, 1991 ) discloses a method of inserting an element that rotates polarization by 45 degrees in a non-reciprocal manner such as a Faraday rotator, in order to compensate for a variation in interference contrast due to the fluctuations in stress-induced birefringence of such a fiber. A general Faraday rotator, however, requires a garnet crystal, and magnetic material that provides a magnetic field for the garnet crystal. It is impossible to provide such substances at the narrow probe tip to be introduced into the patient""s body cavity.
xe2x80x9cIn vivo video rate optical coherence tomographyxe2x80x9d (A. M. Rollins et al., Internet, 1998 Optical Society of America) discloses a method of high-speed scanning of an interference location in OCT, with the reference arm group delay mechanism using a galvanometer mirror. To scan the location at a high speed by rotating the mirror, the inertia of the mirror will dictate that the rotational position of the mirror, if plotted with respect to time, would approach a sine wave oscillation. The depth scan of the interference location is proportional to the mirror rotational position. Because the mirror rotates first in one direction and then in the other, it is difficult to reproduce a two-dimensional image from the obtained interference signals. If the device uses an interference signal obtained by scanning in only one direction, it will neglect half the actually obtained signal data. In such cases, if the doctor continues rotary scanning of the optical system in the optical probe, he or she will provide only half the resolution and dynamic range for a two-dimensional image. Such a decrease in resolution and dynamic range is a problem.
In the above-mentioned high-speed scanning, the use of a resonant scanner, for example, ensures that the time vs. scanning angle curve is a sine wave, not linear. A two-dimensional position and intensity graph, however, may be used to represent an interference signal. Otherwise a shaded image may be used to express intensity as shades, based on the two-dimensional interference locations and detection positions. In such cases, to display interference signals accurately, it is impossible to use the interference signals obtained in time series, because the interference signals are linear with respect to time, and because the interference location is nonlinear with respect to time. A tomogram of the biological tissue inside, for example, may be produced. In such cases, if the device fails to indicate accurately the interference location and the detection position, the image produced will be warped, and it is impossible to accurately measure length with a scale.
As described above, the device may use a nonlinear scanning means such as a resonant scanner. Depending on the scanning angle, in such cases, the Doppler frequency (which characterizes the interference signal produced) varies in proportion to the scanning speed of the interference location. Optical heterodyne detection in OCT uses this Doppler frequency for detection; therefore, it ensures high signal-to-noise ratio. If the scanning is nonlinear, the Doppler frequency may vary. In such cases, if the frequency characteristics of the demodulator circuit are set so that the demodulator can detect all possible interference frequencies in a wide range, then the detection will include excess noise, and result in a decrease in the signal-to-noise ratio. The inferior signal-to-noise ratio is a problem.
If the subject is scanned in a two-dimensional manner, methods of enlarging a part of the displayed image include: a method of cutting unnecessary parts away from that part; and a method of changing the scanning range. Changing the scanning range may require adjusting the speed of scanning the interference location, to the change in scanning range. To improve the signal-to-noise ratio, scanning the interference location may be slowed. In these cases, the Doppler frequency may also vary; therefore, setting the frequency characteristics of the demodulator will pose a similar problem.
As disclosed in U.S. Pat. No. 5,321,501, an optical probe to be introduced into the patient""s body cavity, generally, needs to be detachable from the observation device body for the purpose of cleaning and sterilizing. If detachable, a broken optical probe can be easily replaced with a new one. During assembly, single mode fibers undergo various stresses. Consequently, when provided in each optical probe, each single mode fiber may produce a distinct intrinsic birefringence. Whenever replacing probes, the doctor has to use a polarization plane adjustment means to orient, toward polarization of the reference beam, polarization of reflection that can be obtained by the optical probe, from the subject. It is necessary to maximize contrast of interference with the reference beam. The work is troublesome.
As disclosed in xe2x80x9cIn vivo video rate optical coherence tomographyxe2x80x9d (A. M. Rollins et al.), a galvanometer mirror or resonant scanner mirror may be used for high-speed scanning of the interference location in OCT. Because of the inferior temperature characteristics, the galvanometer mirror and the resonance scan mirror vary the scanning range and scanning speed, depending on the variation in temperature.
As disclosed in the above cited Rollins et al. Publication xe2x80x9cIn vivo video rate optical coherence tomographyxe2x80x9d, a galvanometer mirror or resonant scanner mirror may be used for high-speed scanning of the interference location in OCT. In such cases, in order to maximize drive speed, the mirror will be driven so that the drive curve may approach a sine wave. As described above, the nonlinear drive results in a nonlinear relationship between the interference location and interference signals obtained in time series. Such a nonlinear relationship is difficult to handle. The inferior signal-to-noise ratio, aggravated by setting the frequency characteristics of a demodulator, is a problem.
When displayed on the screen, an OCT tomogram is drawn using optical length, which greatly differs depending on the medium. As the medium, air, for example, has a refractive index n of approximately 1; whereas the biological tissue has a refractive index n of 1.3 to 1.5. When displayed on the screen, optical length is n times the actual length. Such difference causes a significant error, and that is a problem.
The outer sheath with optical permeability is generally made of a resin tube such as fluorine resin and polyamide (nylon). Since the refractive index of the resin tube greatly differs from that of air sealed between the optical element and the outer sheath, significant light reflection occurs at the index of refraction interface between the air inside the outer sheath and the sheath material itself. Similarly, significant reflection also occurs at the index of refraction interface between the outer sheath surface and the air, water or gastrointestinal fluid outside the sheath. The reflection attenuates the irradiation and observation light, thus degrading the S/N ratio for observation.
Since OCT, in principle, detects the correlation between the reflection intensity on the optical path and the optical path length, presence of two faces with intense reflection in the vicinity will cause a degradation in the amount of optical power available for image formation. They will also result in the formation of reflections at the probe sheath surfaces which could be large compared to any tissue reflections, and will thus tend to overwhelm the dynamic range of the detection electronics and make it difficult to display weak tissue reflections on the same screen as the strong sheath reflections. Furthermore, multiple reflections may form between the reflecting surfaces, resulting in the generation of ghost images of the tissue where no actual reflection surface exists. The observation light, which is light scattered and reflected by the living body, is relatively weak light when compared with the above mentioned reflection intensity, the ghost will considerably deteriorate the observation performance. This is especially damaging if the ghost image appears at the same location as the actual tissue image, in which case there is no simple way to separate them.
On the other hand, xe2x80x9cIn vivo Endoscopic Optical Biopsy with Optical Coherence Tomographyxe2x80x9d (G. J. Tearny et.al, Science vol.276) disclosed an OCT optical probe which has a transparent sheath to cover and seal up to the optical element such as the prism at the tip end.
The prior art suffers from problems that when the endoscope with the probe inserted through its insertion opening is inserted into a body cavity or when the probe is bent, the rotating support for the optical element at the tip end comes into contact with the interior of the outer sheath and damages the interior, that optical characteristics of the outer sheath are degraded by irregular reflection of lights at the damaged portion, and that the observation quality deteriorates as the OCT irradiation light emitted by the optical element and observation light reflected by the living body are shielded.
Even when the damage on the sheath does not coincide with the irradiation or observation light position, the damage appears at the location of observation light since the rotary sheath reciprocates against the outer sheath because of the bending shape of the probe.
Furthermore, probe sheaths made from such materials as fluorine resin and polyamide have very poor optical quality, such that they have randomly spaced internal index of refraction boundaries, internal striations, and particulate scatterers which act to degrade the wavefront of light propagating though them and to scatter a significant portion of light into unpredictable directions. Since the probe design depends upon light being directed to a diffraction-limited focussed spot in the tissue and then being re-coupled into the single mode fiber on the return path, any passage through elements which degrade the optical wavefront or scatter light severely degrades the re-coupling of image-forming light into the probe and the heterodyne efficiency of the OCT interferometer. Thus, a probe window of high optical quality would be highly advantageous to obtain the best possible OCT image quality.
As mentioned above, recently an OCT device has been developed for observing tomography structure of a living body by using a low coherence light length of interference. For observing the tomography structure by using the low coherence light, a method is used for dividing light from a light source for generating the low coherence light into signal light and reference light, irradiating the signal light to an observed object, after that, composing the reflected light from the object measured with the reference light again, and detecting the interference light between both of them. Then, as it is possible to obtain the interference light at only the place that length of an optical path of a flux of the signal light is agreed with that of the reference light when changing the length of the optical path on the side of the reference light, it is possible to obtain the same effect as scanning the place for observing the measured object by changing the length of the optical path on the side of the reference light, and to possible to observe the tomography structure of the measured object.
FIG. 1 shows an example that of OCT is applied to an endoscope. The light from a low coherence light source 202 is combined with a single mode fiber 205 and is led to a coupler 204. The light is divided the light into the signal light 206 and the reference light 207 by the coupler 204. The light on the side of the reference light 207 divided is lead via lens 209 to a means 214 (scanning mirror) for changing the optical path 214 by the single mode fiber 205. The light returned from the means for changing the optical path 214 is returned to the coupler 204 by the single mode fiber 205 again.
On one hand, the light divided on the side 206 of the signal light is led to the end optical system 208 on the side of the signal light 206 by the single mode fiber 205 different from on the side of the reference light 207, irradiated to the measured object, reflected from the measured object, returned, therefore, is composed to the light returned from the side of the reference light by the coupler 204 through the end optical system on the side of measuring and the single mode fiber again. The composed light returned on the side of referring and measuring is led to a detector 212 by the single mode fiber 205 and the interference signal is detected by the detector 212.
FIG. 2 is a cross-sectional view showing non-prior art enlarged details of an end part on the side of measuring in the FIG. 1.
The end optical system 208 is constructed at the end of the single mode fiber 205. In the end optical system, a lens unit 220 contains a refractive index distribution lens (Gradient Index lens: GRIN lens) 221 for gathering the light to a living body, a Faraday rotator 222 for canceling the influence of polarization caused by bending of the single mode fiber 205 and prism 223 for changing the direction of the light. Further, the cylindrical transparent sheath 225 covers the outside of the lens unit. As shown by arrow 219, the single mode fiber 205 and the lens unit 220 are rotated as the longitudinal direction (the central axis of the cylindrical sheath 225) is an axis. As above mentioned by rotating the mechanism, it is possible to observe the measured object not only in the cross-sectional direction but also radially.
In the above described related art, the end optical system 208 has a problem in that after light outgoing from a single mode fiber 205 is reflected several times on the boundary surface of a optical elements such as a Faraday rotator 222, a refractive index lens 221, prism 223 and sheath 225 and returned to the single mode fiber 205 again, an image of a living body and ghost overlaps and quality of the picture is degraded.
The reasons for that are described in reference FIGS. 3(A)-3(E).
FIGS. 3(A) through 3(E) show an example of causes for generating ghosts, and behavior of the light outgoing from the single mode fiber is schematically shown as time passes.
FIG. 3A: When the light outgoing from a single mode fiber 205 comes to the boundary surface between the refractive index distribution lens 221 and a Faraday rotator 222, part of the light, light a, but part b penetrates is reflected rearwardly toward the single mode fiber 205 again.
FIG. 3B: Light b is then again reflected by the rear surface of the refractive index distribution lens 221 and is directed toward the object. Light a, however, passing as it was in the FIG. 3A, proceeds through the Faraday rotator (FR) 222 and the prism 223 to the middle of the sheath 225 and the tissue 226 of a living body.
FIG. 3C: Light a is reflected in tissues 226 of a living body and directed back toward the single mode fiber 205. Meanwhile, light b is proceeding though the refractive index distribution lens 221 to the side of the object.
FIG. 3D: Light b is reflected on the boundary surface between prism 223 and FR 222, and is directed back toward the single mode fiber 205 again. Then, light a arrived at the boundary surface between the prism 223 and the FR 222, and overlaps light b.
FIG. 3E: Overlapped light a and light b is return to the single mode fiber 205.
In case of the conditions above mentioned, the signal light from the tissue 226 of the living body and light b reflected in the end optical system several times overlap, and an image when seeing the tomography structure of the living body is that the image 228 (FIG. 4) of the ghost due to light b overlaps the image 227 of the tissue of the living body.
The image 228 of the ghost is often remarkable when the number of times of reflection on the end surface of the optical element is less than three times. Generally, as strength of reflection per surface of the optical element is fromis xe2x88x9220 dB to xe2x88x9230 dB relative to the incident light, when the number of times of reflection on the end surface of the optical element is three times, total strength of reflection is from xe2x88x9260 dB to xe2x88x9290 dB, and when the number of times of reflection is four times, it is from xe2x88x9280 dB to xe2x88x92120 dB. On the other hand, strength of the signal from the living body is xe2x88x9250 dB to xe2x88x9270 dB. Therefore, when the number of times of reflection on the end surface of the optical element is even less than three times, it can be almost the same level as the strength of the signal of the living body and causes difficulty in observation.
The path of the reflective light as above mentioned is an example; multiple reflection in the real end optical system occurs in various paths except besides the path as above mentioned. Particularly, there are problems when the shape of the sheath is cylindrical because it is difficult to treat to prevent reflection from the surface of the sheath, and it is easy then for light reflected three times to overlap on the observing position.
One more problem of the above mentioned end optical system is the structure of the end optical system in that the light outgoing from the single mode fiber is reflected on the boundary surface of the optical element and the reflected light is directly returned to the single mode fiber 205. If the such structure as is called the structure of once reflection, xe2x80x9cstructure of once reflectionxe2x80x9d, in the optical system, the structure of once reflection will reflect the reflected light from all the boundary surfaces except the end surface on the side of the fiber of the refractive index distribution lens.
If the structure of the optical system is the structure of once reflection and the light reflected from the end surface of the optical element is incident on the single mode fiber, unnecessary light will return to the single mode fiber and an S/N ratio when detecting the interference signal will be significantly reduced. Therefore, if the structure of the optical system is the structure of once reflection, it will be hard to see an observing screen.
FIGS. 5A and 5B show an example of the structure of once reflection that the light is reflected on the boundary surface between a Faraday rotator 222 and prism 223, wherein FIG. 5A represents the actual optical path, and FIG. 5B represents by folding the optical system from the reflected surface so that it is easy to understand the optical path. It is known from FIG. 5A that the reflected light returns to the single mode fiber 205 again. In the optical system, the light reflected by another boundary surface returns to the single mode fiber 205 again similarly to the example as above mentioned.
Next, FIG. 57A is a schematical view showing an end optical system of an optical scanning probe from the side of en end and FIG. 57B is a schematical view showing from a side surface. As shown in FIGS. 57A and 57B, the end optical system includes a transparent Teflon tube 320, a prism 308, a GRIN lens 311 and a single mode fiber 301. Light guided into the single mode fiber 301 is incident on GRIN lens 311, prism 308 and Teflon tube 320 and becomes an observing beam 317. The Teflon tube 320 has a concave lens effect in the peripheral direction of a cylindrical surface of the tube. Therefore, a focal distance 319 a in the peripheral direction to a sheath cylindrical surface of the observing beam 317 is longer than a focal distance 319 b in the longitudinal direction of the sheathe.
In the related art shown in the FIGS. 57A and 57B, relation of positions between the focal position 319a in the peripheral direction and the focal position 319b in the longitudinal direction to the sheath cylindrical surface of the observing beam 317 is fixed and impossible to change.
In view of the above problems of the related art, an object of the present invention is to provide a optical imaging device to compensate for a variation in interference intensity, due to the variation in birefringence of the fiber in an optical probe, even if the optical probe is bent in a patient""s body cavity. Further, the optical imaging device provides a physically small birefringence compensation means such that it can be included at the optical probe tip. Further, the device compensates for a variation in interference contrast, due to the variation in birefringence of the fiber, caused by every rotation of the bent and turned system. Further, the device makes the birefringence compensation means of the Faraday rotator function effectively. Further, the device provides a physically small composition of the distal optical member for the optical probe.
Another object of the present invention is to provide an optical imaging device to obtain an interference signal corrected so that scanning can be regarded as performed in the same direction, even if the scanning direction oscillates due to the swing of the swing mirror in the propagation delay time-varying means. Further, the optical imaging device prevents the corrected interference signal from deviating, even if slight hysteresis or deviation, during scanning to and fro, is caused by the hysteresis of temperature characteristics and dynamic characteristics, when the device uses a high-speed scanning mirror, such as a galvanometer mirror and a resonance scan mirror in particular. Further the device performs the above-mentioned correction automatically.
Furthermore, an object of the present invention is to provide an optical imaging device to obtain an interference signal which is linear with respect to spatial distance from the probe tip, even if scanning the interference location is nonlinear with respect to time.
Furthermore, an object of the present invention is to provide an optical imaging device to enable the demodulator to detect the envelope of the interference signal with a high signal-to-noise ratio, even if the Doppler frequency of the interference signal is varied by the variation in speed of scanning the interference location.
Furthermore, an object of the present invention is to provide an optical imaging device to adjust the plane of polarization automatically.
Furthermore, an object of the present invention is to provide an interference location scanning means which has a scanning range and scanning speed that are difficult to disturb from the outside.
Furthermore, an object of the present invention is to provide an interference location scanning means which has the ability to perform high-speed scanning that is nearly linear.
Furthermore, an object of the present invention is to provide an optical imaging device to measure length on the screen correctly, whatever the refractive index may be.
Furthermore, an object of the present invention is to provide means for attenuating the reflection on the inner or outer surface of the outer sheath.
Furthermore, an object of the present invention is to make the inner side of the outer sheath resistant against damage by the rotating optical element and to have a sheath with good optical quality in the region of the probe where signal light is transmitted.
Furthermore, an object of the present invention is to provide an end optical system on the side of the signal light of OCT in which ghosts do not appear, the S/N ratio is good, and the ability to observe is excellent considering the problems as above mentioned.
Furthermore, an object of the present invention is to provide an optical scanning probe for advancing the S/N ratio.
Furthermore, another object of the present invention is to provide the optical scanning probe in which the observing depth degree is large
Furthermore, an object of the present invention is to provide the optical scanning probe with optics that are easy to assemble and manufacture.
According to the present invention, variations in interference intensity, due to the variation in birefringence of the fiber in the probe, are compensated for, by the provision of a polarization compensation means on an optical path, from the emission end of the single mode fiber in the optical probe to the subject. Further, a physically small polarization compensation means can be provided at the optical probe tip, because the Faraday rotator uses a magnetic garnet crystal. Further, almost all substantially parallel rays incident on the Faraday rotator will cause the plane of polarization to rotate by 45 degrees accurately, to produce high efficiency in polarization compensation.
Further, according to the present invention, the device has a first memory means for preserving an interference intensity signal that corresponds to a particular one-way swing based on the detection by the reference position detection means, and a second memory means for preserving an interference signal that corresponds to a swing in the opposite direction to the particular one-way swing. Backward reading of data stored in the first memory means and forward reading of data stored in the second memory means produces interference signals that indicate scanning in the same direction. Further, different delays are provided for first memory means and the second memory means so that data, stored in the first memory means and the second memory means, may be read at delayed points of time. Controlling the delays correct hysteresis and deviation caused by slight differences in the scanning directions to and fro. Further, a reference signal is detected in each interference signal, and the delay setting means is adjusted so that both reference signals coincide with each other at the position of the reference signal. It is possible, thereby to correct automatically hysteresis and deviation caused by the to and fro scanning.
Further, according to the present invention, the use of a memory means for preserving interference intensity signals in time series, and a calculation means for calculating a memory address that corresponds to the interference location, to calculate the address, and to read data stored in the memory, produce an interference signal that corresponds to the interference location.
Further, according to the present invention, the device uses a calculation means to calculate the Doppler frequency of an interference signal produced by scanning the interference location. Changing the frequency characteristics of the demodulator, according to the calculated Doppler frequency, can constantly set the demodulator, to produce a high signal-to-noise ratio.
Further, according to the present invention, the device uses reference reflection means provided close to an end of the optical probe insertion unit, and obtains reflection data from the reference reflection means, in the form of an interference intensity signal produced from the interference means. Only setting a polarization adjustment means so that the interference contrast signal may be maximized, generally, is needed to adjust the plane of polarization for each probe automatically.
Further, according to the present invention, the propagation delay time-varying means has a dispersive means, imaging means, and reflection mirror. The reflection mirror includes a polygon mirror. The rotation of the polygon mirror enables scanning the interference location. It is easy to stabilize the scanning range and scanning speed, because the polygon mirror scanning is nearly linear with respect to the time series, and because it is easy to stabilize rotation speed.
Further, according to the present invention, when the reflection mirror is scanned by a resonant mechanical scanner, the scan becomes more linear by providing a specialized drive signal including one or more additional higher frequency harmonic components.
Further, according to the present invention, the optical imaging device comprises one or more scales that include a scale indicating an optical length in medium, or a scale indicating an optical length in the tissue, or a scale indicating optical length in the medium in those regions of the image representing the medium and which indicates optical length in the tissue in those regions of the image representing the tissue.
Also, a rigid light permeability part made from an optical quality material such as glass or fused silica has no internal index of refraction boundaries, striations, or scatterers, and can be manufactured or polished in such as way as to provide a good optical surface on the inside and the outside.
Further, according to the present invention, as light that the number of times of reflection on the end surface of an optical element of an end optical system on the side of signal light of an optical tomography diagnosis device is less than three times does not return to a single mode fiber, it is possible to obtain the end optical system on the side of the signal light of the optical tomography diagnosis device that ghost does not appear and a S/N ratio is advanced.
Further, according to present invention, by matching agreeing the focal position in the peripheral direction of the cylindrical surface of a sheathe with the focal position in the longitudinal direction of the sheathe and by concentrating the focal point to a point in an observing beam outgone from the optical scanning probe, an optical scanning probe for advancing the S/N ratio is provided.
Further, according to present invention, by making the beam spot diameter of an observing beam almost uniform over a long distance in the direction of the major axis of the observing beam., the optical scanning probe that observing depth degree is large is provided.
Further, according to the present invention, by placing reflection reduction coating inside or outside or both of the probe sheath, the reflection on the border of inner or outer medium to the sheath is reduced and thus the ghost and the light intensity reduction caused by reflection is avoided.
Further, according to the present invention, by placing a rigid light permeability part on the optical window where an optical emitter and receiver are located, the optical characteristics of the outer sheath are not degraded even when the rotating support for the optical element at the tip end comes into contact with the interior of the outer sheath. Also, a rigid light permeability part made from an optical quality material such as glass or fused silica has no internal index of refraction boundaries, striations, or scatterers, and can be manufactured or polished in such a way as to provide a good optical surface on the inside and the outside.
Further, according to the present invention, as light does not return to a single mode fiber when the number of times of reflection of light on the end surface of an optical element of an end optical system on the side of signal light of an optical tomography diagnosis device is less than three times, it is possible to obtain the end optical system on the side of the signal light of the optical tomography diagnosis device so that ghosts do not appear and the S/N ratio is advanced.
Further, according to the present invention, by matching the focal position in the peripheral direction of the cylindrical surface of a sheath with the focal position in the longitudinal direction of the sheath and by concentrating the focal point to a point in an observing beam outgone from the optical scanning probe, an optical scanning probe for advancing the S/N ratio is provided.
Further, according to present invention, by making the beam spot diameter of an observing beam almost uniform over a long distance in the direction of the major axis of the observing beam, the optical scanning probe that observing depth degree is large is provided.